1. Field of the Invention
The present invention relates to a method of recording information such as video or audio signals, data of electronic computer or the like in a recording medium formed on a preselected substrate with the aid of laser ray or beam on a real time basis. In particular, the invention is concerned with a recording method of information in which phenomena such as thermal evaporation, melting or displacement of material constituting the recording medium under the action of the laser beam or ray are utilized for the recording.
2. Description of the Prior Art
In recent years, attention has been increasingly paid to the information recording method in which information is written in a thin film of metal or the like formed on a substrate by using a laser ray or beam. According to such a method, the information recording has been accomplished by forming holes or recesses in the metallic thin film under the action of a thermal energy a beam such as laser ray.
There have been already published some reports concerning the fundamental methods of recording and reproducing information in or from a recording disk by using light beams. For example, reference is to be made to a literature entitled "A Review of the MCA Disco-Vision System" submitted by Kent Broadbent in "the 115-th SMPTE Technical Conference & Equipment Exhibit" held on Apr. 26, 1976. In the first place, a brief explanation will be made on the principle of such known method for a better understanding of the invention.
Referring to FIG. 1 which illustrates a principle of the method of recording information on a disk by means of a light beam, the disk denoted by reference numeral 1 comprises a substrate 2 which is usually formed of glass or the like material and a thin film 3 of recording medium deposited on a surface of the substrate, as described in detail hereinafter. The disk 1 is adapted to be rotated at a high speed through a rotatable shaft 4. There is disposed over the disk 1 with a predetermined short distance spaced therefrom a lens 5 through which a laser beam 6 which has undergone a pulse-like modulation depending on the information to be recorded is focussed onto the recording thin film 3 for irradiation thereof. Consequently, the portion of the recording thin film which has been irradiated with the laser beam 6 is heated to be molten and displaced or evaporated. The size, configuration and position of hole or recess thus formed in the recording film and having usually a shorter diameter on the order of 0.5 to 1.2 .mu.m will correspond to the information carried by the applied laser beam 6. In this manner, information such as video signals, audio signals or the like can be stored in the recording film as the corresponding holes or recesses, which holes or recesses may or may not penetrate the entire thickness of the recording film. When the stored information is to be reproduced, the disk 1 is rotated at the same high speed as in the case of the information recording and at the same time a reading-out light beam such as a laser beam is focussed and projected on to the recording film. Through detection of intensity or the like characteristics of the resulting reflected ray, it is possible to determine the presence or absence of holes and recesses, positions, sizes and shapes thereof, thereby to reproduce the recorded information.
There have been hitherto known various types of recording mediums which can be used for the recording film in the information recording method described above. As typical examples, recording medium of Bi (refer to Japanese Patent Publication No. 40479/1971), recording medium of Bi-Se in Bi/Se ratio of (2.5.about.3.5)/1 (refer to Japanese Laid-Open Patent Application No. 87304/1975), medium of chalcogen glass (Japanese Laid-Open Patent Application No. 42849/1975), and a medium containing crystalline chalcogen as the main component (Japanese Laid-Open Patent Application No. 51733/1975) may be enumerated.
Hitherto known materials for the recording medium as mentioned above are not yet used at present in practical applications for the reasons that the signal-to-noise ratio of the read-out signal of the information which has been stored with a high density is excessively low, the information as written-in tends to become unstable, and so forth. For example, in the case of the recording medium of Bi or Bi-Se, it is known that the shape of recesses formed through the irradiation of the laser beam will often become irregular. Under such situation, the information as read-out will contain a large proportion of noise components and thus the signal-to-noise ratio higher than 25 dB will hardly be attained. Even when the recording is made on the same recording medium, the irregularity in the shape of the information recess has a tendency to exert more undesirable influences on the signal-to-noise ratio, as the size of the recess becomes similar. Such irregularity in the shape of the recess is in most cases ascribable to the fact that, upon melting and outward displacement of material of the recording medium as caused through irradiation of the focussed energy beam, a part of the molten material remains in the recess in a spherical form due to the surface tension and/or in a form of particle due to partial crystallization. In such case, the size of the spherical remainders and the crystallized particles will remain unchanged even if the size of the information recess is relatively decreased. Thus, the signal-to-noise ratio is subjected to more adverse influence, as the dimension of the information recess or hole is more reduced.
For the similar reason, it is practically impossible to attain a desirable signal-to-noise ratio in the case where the chalcogen glass or the crystalline chalcogen substance is used as the recording medium.
After many and various experiments, the inventors have discovered that a recording medium made of chalcogen glass and in particular amorphous chalcogen glass can exhibit a good recording performance or characteristic when chemical composition thereof is selected in an appropriate manner, as is disclosed in the specification of abandoned U.S. Patent Application Ser. No. 706,247, filed July 19, 1976, of the inventors of the present application as co-inventor with another. In this connection, it is however to be noted that the composition for the recording medium disclosed in the above U.S. patent application can exhibit desirable characteristics only when the recording of information is made immediately after the formation of the recording thin film and the reading-out of the information is carried out within a time span during which the recording medium is not yet subjected to variation in its characteristic. For example, when the recording medium as formed is exposed to air at a room temperature, the signal-to-noise ratio is significantly decreased with lapse of time. There, in practical applications, the decreasing of the signal-to-noise ratio involved significant problems on the reliability or accuracy of the information.